This invention relates generally to a method and apparatus for remote imaging of objects enveloped by a backscattering medium which is at least partially transmitting to light. More particularly, this invention relates to a method for detecting and imaging underwater objects from an airborne platform using a novel imaging lidar (light detection and ranging) system.
There is a continuing need to develop methods of detecting underwater targets from remote airborne locations and over relatively short time periods. This is of particular importance in the case of certain military applications where, for example, the detection of moored mines from helicopters and other aircraft is vital to ensuring safe seas. Presently, cumbersome and time consuming wire line devices must be used. These devices are lowered into the water and of curse, are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging.
Fortunately, an improved and novel system for remote detection and imaging of objects underwater (or objects obscured by other backscattering media which is at least partially transmitting to light such as ice, snow, fog, dust and smoke) from an airborne platform has been described in U.S. Pat. No. 4,862,257 and U.S. patent application Ser. No. 256,778 filed Oct. 12, 1988, both of which are assigned to the assignee hereof and incorporated herein by reference.
The imaging lidar system of U.S. Pat. No. 4,862,257 utilizes a laser to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. Intensified CCD (charge coupled device) cameras are electronically shuttered after a time delay corresponding to the round trip propagation time to and from the target. This timing eliminates light scattered by the water from in front of and behind the target. As a result, the veiling luminance of the water is greatly attenuated and faint target signatures can be seen. The resulting gated images (displayed on a CRT) have sufficient spatial resolution to classify and/or identify the target. U.S. application Ser. No. 256,778 relates to an imaging lidar system intended for night vision.
An important feature of the imaging lidar system of U.S. Pat. No. 4,862,257 and U.S. application Ser. No. 256,778 is the timing control method wherein a glint detector is used to trigger gating of the intensified CCD cameras. This timing control scheme is described in detail with reference to FIG. 5 of U.S. Pat. No. 4,862,257. Referring now to both FIGS. 1 and 5 of U.S. Pat. No. 4,862,257, it will be appreciated that an important feature of the prior imaging lidar system is the short duration, electronic shuttering of intensified CCD camera 18 and/or 20 after a time delay corresponding to the round trip propagation time of a short laser pulse between the "system" and a reflecting target or scattering volume of interest. The method for establishing this time delay is as follows: A photodiode or photomultiplier 37 (see FIG. 1) detects the glint return of laser pulse "n" from the water surface. The detector signal is conditioned in amplifier 35 and used to initiate trigger of delay generator 22; the end objective being to gate the camera on (i.e. open the shutter) for the desired return of the "n+1" (next) pulse. Therefore, the total delay required is the pulse period (e.g., 67 msec for 15 Hz or 50 msec for 20 Hz laser pulse repetition frequency), plus desired water depth delay, less system delays. It has been determined that the system delays (camera response time, delay generator response time, etc.) typically exceed the desired depth delay, thus necessitating the one pulse delay. Since timing is initiated by the surface glint return, the platform altitude is always compensated (relying, of course, on the fact that any change in altitude during one pulse period is insignificant). The fact that total delay can be less than the pulse period requires that two delay generators 22 and 24 be used in series. A requirement for this timing method of U.S. Pat. No. 4,862,257 is that the frequency of the outgoing laser pulses must be extremely stable. This imposes critical requirements upon the accuracy of the system's master clock and the jitter of the laser firing electronics referenced to the laser trigger input. The result is that the time delay method of the prior imaging lidar system is relatively complicated and therefore subject to error.